When you're thinking of your new stocking plan keep in mind that the molly takes up much more space than the original guppies you had intended. If it were my tank this is what I would do (I'm basing this around your desire for kuhlis and your current stock):
current:
1 female betta
1 female molly
future:
4-5 kuhli loaches
1-2 ADF (African dwarf frog)
(these two take up your bottom pretty much, so I wouldn't really go for ottos)
Now keeping in mind that mollies seem to do better in higher temps, around 78-80 and that's about where the ADFs like it as well, and the kuhlis will fit well there, you should be ok. But you'll want to keep future options in mind as far as temperature goes and size. I would recommend in that size tank either: more mollies (but you'll end up having to figure out what to do with babies), more female bettas (often called a sorority), or a small school (6ish) of a smallish tetra/barb that likes the warmer spectrum. I wouldn't add any more bottom dwellers as you'll quickly run out of territory and feeding space an though it may not look like it the bioload will be full. Again if it were my tank I would do a school of
6 (maybe 8 but that might be pushing it) cardinal tetras (they like this warmer temp)
or
4-6 penguin tetras (warmer temps are enjoyed by them too)
or
6 cherry barbs (I REALLY like them cuz the males are red and females a very pretty bronze/gold)
All of these should be left alone by the molly as they are big enough to not be food.
If you decide to go with more mollies add either 3-4 females, or 1 male and 2-3 females. If you add more female bettas add 3-5 and add them all at once to suppress any territorial fighting.
Again this is just if it was my tank what I would do, there are plenty of other options out there