I know you said no cichlids. Angels and rams (or kribensis) though make excellent tank mates until they breed. Angels would make quick work of the platy babies and eliminate any over population problems.
Other live bearers like guppies, dwarf live bearers,enders, and swordtails (closely related to platys).
Think about a layered approach to your tank. Platy are top and mid level fish. 40 is just big enough for a bristlenose pleco for a bottom feeder. But you might perfer a fish that moves in a group, so Otto cats, Otocinclus Catfish (several species, but much the same behavior) like living in groups of 5. They are better algae eaters then are cories. They are not shy and play on the glass all the time also.
Cories are also another natural choice.
Bettas and live bearers are usually bad combinations. Each betta is slightly different in personality. I have one that just barely gets along with 4 guppies in a 55 gallon tank. You might try it. Oh, the reason I said betta, while they will swim and eat at all levels, their main hang outs are near the top.
There are lots of small barbs and tetras for the middle levels to join your platy population.
If you want a big fish, a butterfly fish would be a nice surface fish.
http://en.wikipedia.org/wiki/Freshwater_butterflyfish
With the tank heavily planted they can get along with a wide range of fish. Too lightly planted and no more platy.
Run any stock though a stocking calculator line aqadvisor.com
They will tell you of any overstocking issue, amount of water change, and any problems between species.