Bala sharks (silver sharks) grow quite quickly and a 3 footer won't be enough. They need a 125 gallon minnimum and are a schooling fish so the minnimum number you should ever keep is 3. They can get to about 14". You have to re-home this fish with others of its kind in a suitably large tank.
If the pleco is a common, it can get to 18". The same applies to sailfin/gibbiceps plecs which are also quite often available. They grow very slowly though so you have time before it needs to be moved.
The betta, though they shouldn't be with male guppies, can be kept in a community provided it won't be out-competed or nipped. I would actualy say it would be ok with all but the guppies and the silver shark (silver sharks, as I said, are big schooling fish and so are easily made nervous which can lead to all sorts of problems when it comes to a small, long-finned fish - they are also very fast when it comes to competing).
Can I also say that actualy you have more than 28" of fish (even without the silver shark and plec) as each molly will potentialy grow to 4" (a lot of people under-estimate their potential size) and that's already 24" of fish! That's realy all you should have in your tank strictly speaking (though provided you maintain the tank well and do your weekly water change and gravel vac. and have good filtration the platies - 1.5", guppies - 1" for males, up to 2" for females - and cories - peppered get to 3" - are just about ok). I don't count the betta as you said you're moving him soon.
