Howdy! General newbie questions...

Wow that's a long post.

Here's my opinion on some of this.

We would really need more information on your cleaning habits to really know what may be keeping your fish that small. Is it only a a few inches from the tips of the fins, or just to the circular main body part? Clean fresh water is pretty important in any tank, and if you go long periods without partial water changes, the toxins that build up may have something to do with the size, but really we would need to know more.

The algae may also be a result of cleaning habits. Of course it could just as well be caused by excessive light, overfeeding, water conditions, ect. Again, more info please.

Heaters: Heaters are designed to turn themselves on and off to maintain water temp. There are lots of them on the market and most are preset to an ideal temperature for tropical fish. Of course some require warmer or cooler conditions, but that depends on the species. Still, an inexpensive heater should serve as well as a pricy one, but may be subject to failures. Do a product search for aquarium heaters on google or another search engine and do a bit of research.

Filters: Again, there are many types of filters for different budgets. One of my tanks has a Emperor dual biowheel HOB filter, and another a cheap whisper HOB, and they both do their job wonderfully. Normally I don't worry about buying cartridges of any type on a regular basis as I don't use carbon filtration in my filters. Just filter floss that gets rinsed out in dechlorinated water once in a while, and replaced when it's worn out, but that can be maybe only 2-3 times per year. If you are going to use carbon or other materials in your filter, you're going to have to get used to buying replacement components. The ideal setup would be to have an ecosystem established that requires little interference from you or your chemicals. So do a search for those products too and research. Hang On Back filters can be noisy and obtrusive, but they are also pretty inexpensive and reliable. Canister filters will be out of sight if you place it right, but can be costly. Undergravel filters, sponge filters, ect may be other options. Just do your research and find what works best for your situation.

As for fish, well that's a personal choice. With an angel in the tank, mind your tankmates if you add more, but honestly I don't know that you have a whole lot of room. The angel would probably prefer tankmates that are pretty calm. Nothing nippy, so barbs are out. Rasboras would make a nice addition. They are attractive (in my opinon) they are pretty laid back and easy maintenance. They are also fairly resilient. The only larger fish you might add is another angel to provide some company, but don't push the limits of your tank.

Hope this helps, but please don't take it for gospel. Do your research and get info from different sources.

Remove the pictus cats if you want.

Add another angel to company the one you have.

And add a pleco for algae control.

That should be good.

For a filter I like Aqua Clear filters. They are generally inexpensive and provide a nice large compartment for media. I then buy an extra aqua clear sponge and add it instead of the carbon bag (2 sponges). From that point on I only rinse the sponges in the old tank water that I remove during water changes. I have been using the same sponges for over 2 years now. I will replace them when they fall apart. As this is where most of the good bacteria lives it is never a good idea to change them out altogether. Save the carbon bag for when you need to remove medications or water discolorations.

Since it is a smaller 29 gallon tank I would be careful about adding another angel. They get big and if they are not compatible might start to fight very easily. And if they form a breeding pair could terrorize the tank.

An algae eater is fine to get (not a common pleco though as they get far too large) but getting to the route of the problem with algae is best. First don't tear down the tank and clean it if possible as it sets your cycle back to zero. Get a magnetic algae scraper that will clean the glass and use it every week with your weekly water change. And if you haven't been doing them change 20-30% of your tank water each week. That will keep tank nutrients low and slow down algae growth. And when doing your water changes use a gravel vacuum. They are cheap and work by siphoning the water out of the tank into a bucket and can suck dirt etc.. right out of the gravel bed. I also like to stir the top layer of my gravel as this flips it over and reduces the algae I get on the top layer of gravel.

Buy a heater, your fish need the stability. Tronic heaters are well thought of, but cost a bit more. Tehy are worth the cost though as you found out from a cheaper heater.

As for the angel fish. Well it does sound like it is stunted. That is probably (just guessing) due to poor water quality (heat, nitrates etc...) rather than a lack of food. That is another reason I wouldn't add another angel as this one would probably get picked on.

If you like catfish then try trading your pictus in for some cory catfish, they stay very small and you could have a nice school of 5 or so. Then look into rosey barbs. They get a few inches long and the males a a nice pink colour and they school. So get 5-6 of them. And possibly look into adding some plants and leaving that light on for 10 hours a day. Fish need a true light cycle just like people do.

Algae eaters may be ugly...but too much algae can be uglier. I use one large mystery snail (Gary) in my 20L and he seems to keep things pretty clean. If my algae became a problem I'd get an Algae eater in a heartbeat. Good luck.