If you really want to raise fry, but don't have a tank available solely for this purpose, then you're pretty much stuck with having to just leave things to chance and let nature take its course.
You'll end up with more fry losses, but the ones you do end up with will be healthier as a result.
The fry do much better when allowed to swim freely from the moment they're born anyway (i.e. as opposed to being raised in a breeding trap). They are forced to fend for themselves more...becoming stronger swimmers by having to evade hungry adults and by venturing out into the open to find food.
Back when I raised platies, the fry were raised in tanks containing adult fish. Some inevitably perished, but many of them survived on their own by hiding in dense plants. Some also developed the tactic of hanging around my clown pleco in order to keep away from the adult platies (who wouldn't go near the plec).
By the way, I checked out your tank in the rate my tank section...it looks great! In fact, the color-scheme is such that a school or two of tetras---I'm thinking lemon tetras and/or silver-tip tetras---would look great against the darker substrate!
Big Vine