Eleanor, I'm replying more as a bump (to the top of the forum for members to see) than because I have anything particularly helpful to offer.

I grow quite a few Peperomia and thanks to gifts that friends have brought over, I've grown a two or three Hypoestes--grown, past tense, being the operative word. :-) Well, grown would actually be an overstatement...

I don't think the two have similar water needs, not that I ever figured out what the H actually wants. Treating it like Peperomia (basically as a succulent) doesn't work; treating it like a fern (flooding) doesn't work. They go from cute to hideous practically overnight a few weeks into the game for me.

Light-wise, they should be similar, both liking indirect bright light, but again, not that I really have any clue what light the H wanted before they croaked. lol

I presume your pot has drainage holes? I'm not sure why you layered the soil like a terrarium when, well, it's not a terrarium but perhaps drainage is the reason? If there are no holes, that isn't going to end well.

Keeping it small and fertilizing go against each other in my mind, but I know nothing about bonsai and plants need nutrients, so....Al (tapla) is our bonsai master, and he can assuredly tell you how to fertilize but keep it from growing large. I'd imagine it's a matter of letting it grow and pruning it to shape and ohhhh, believe you me, the H didn't like that either, haha.

Hope you get some help! :-)