I give myself very good advice, But I very seldom follow it. That explains the trouble that I'm always in.
If I don't build a development but do use either colony or military convoy to place a second world, do I get a rebate after placing the first world? Due to not having also built a development?i'm not sure if by "rebate" you mean a.) the bonus card you draw for having placed no development this round or b.) the discounted cost from Colony Convoy.
the rules state that "If a player drew a card for not placing a development, this card could be placed as a second world." which clarifies that a.) yes you can draw that bonus card (assuming you didn't play Convoy development this round). if you're asking whether you can b.) take the -2 discount for Colony Convoy for your first planet... no, "it cannot provide a discount for the first world placed in a round." also, note that they must both be military or non-military planets, "but not more than two worlds."