Pregnancy with them works differently in game, they just spawn a Slime Minion instantly. Though, they can also drop a Slime Core if you defeat them, which lets you summon a Slime Minion of your own. You can also very rarely get a Slime Core from them if you lose to them and satisfy them enough through sex.
Though, if I ever do implement a Slime Heroine, she will likely have a more standard like pregnancy, with the belly and all.
There is no Sex Scene for defeating them. Currently, the only Monster Girl that has a special scene when you defeat her is a Werewolf, the rest don't.