It sounds like the shammy would be a better raid fit for you guys, so I'll mainly address Sliptide. He's currently resto/enhancement. Given he's your only current mail wearer, you can expect his offset to gear up more swiftly than any other healer's offset. Between Avrus and Rose, your melee will surely be your super DPS. Therefore, Sliptide seems like the natural choice to fill the flex-healer spot. That is, you'll probably need his healing on the beasts, but his DPS on other bosses (especially as they become farmed). In DPS races, it usually pays to have a healer go DPS, and if that heal/DPS also majorly buffs your top DPS... well, he's a shoo-in. I also think that offering him the flex-healer would assuage any dissapointment in not taking his warrior Slipfox.
If you took Slipfox, he would want to tank. Tanks know very very well how hard it is to find a tanking spot in a raid, and I think Slipfox knows that by offering an alt who is both a shaman and a healer that his tank is even more likely to lose out vs raid composition. However, if you do have room for him in a tanking role he is a perfectly capable tank.