You're right, if a bot reproduce at 5000 nrg, you keep the body of all bots down by shooting them under 1000 or something. And by reproducing they kee a low body.
That works if there isn't a gene or genes that connect nrg with body, using nrg to make the body at all time bigger than 1000, would kill probalbly all bots of the specie.
If I understand it right you want to kill all bots that have a body above 1000, so keep shooting if the body becomes lower and killing it, that could be a problem let the bot in fact shoot a bot that is weaker, and don't shoot natural strong bots. Or do you want to just keep the body low without killing.
If you want to use this to get weaker opponent, I am not sure if it will work mine multiply for example has a low body, raticus(or something, the one from jez I thought) an extreme high one. Multiply wins becouse of higher numbers, something like ten multiplys are needed to kill one raticus, but it wins.
There are different ways to evolve, you could make extreme heavy strong bots. Or alot smaller ones, most evolutions I run the sim prefers just the smaller ones with joung ages and fast reproducing-time. Some cases almost cancercious. Seldom there repro-time takes longer.
Of cource there are more ways to evolve then body or number, poison or infoshots or ties or anything else.