I constructed a quick tally of the types your Pokemon will be weak against. The I are a single weakness and the O are a double weakness (i.e. dragon/flying being weak against ice twice).
In other words, ground, rock, and ice are the three types you need to watch out for. Breloom can take care of ground Pokemon, Lanturn can deal with rock Pokemon, and Rapidash can deal with ice types, so you're fairly well covered!
In terms of how powerful each Pokemon is compared to other Pokemon of that type... I don't know much in that realm since I usually use whichever Pokemon I like the most. xD