Summons are dependent upon what lineups you are using. The explanation I use, is that if you have a combo where you thought "if only I had a chase of Knockdown into High Float, this would be amazing," that is where summons come in. Each summon can either link your combo or extend them. You'll have to figure out and try to see which one fits your line up best. (The summon I most commonly use for Fire main is the Kamatari summon)