Corys are not invertebrates. They have a skeleton. They are, however, scaleless fish - they have bony plates instead - and this means they can be very sensitive to a number of medications. ALL plecos are also scaleless, and will be just as sensitive to any medications or chemical substances as corys.
Any pleco of any kind (even very small ones, like Clown Plecos) produce vast quantities of waste. This is why they are often referred to as "poop machines." They will foul the water quickly and thoroughly, ESPECIALLY in a small tank. They require very powerful filtration to sift out the amount of solid waste they produce.
I'm not sure what your budget is, or what you consider "too expensive" for filtration, but you will need something powerful enough to turn over the water volume at least 10 times per hour, preferably more, for a pleco. Also keep in mind that the flow rates of all filters are measured WITHOUT any media in them. Once you add the media, the flowrate is often cut in half.
I've never kept angels, but it is my understanding that do not like currents on the level you'd need to manage the waste produced by any pleco. I also would not mix an angel with a pair of Blue Rams. A 29g is too small for all three of those fish, and the Rams will get aggressive and territorial when breeding.
Silver Dollars need to be in groups, and even a single Silver Dollar is far too large for 29 gallons.
Lastly, this is honestly not meant to be rude, as I am sure your friend is a lovely person on many levels, but that tank is INSANELY overstocked. I would not recommend such a list of fish for a 29 gallon tank under any circumstances. That is absolutely cruel to the fish.