Did a few runs on Suicidal 6-man and can safely say the best combo is to simply stick to your specialized weapons: MAC10/FT.
MAC10 should mainly be used against Bloats and Husks (Although unless you have a reliable competent Sharpshooter the two might be secondary priority). FT still conquers the Crowd Control (CC) when things get out of hand and your teammates need to reload. Just follow that Firebug thread (nutterbutter thread made) on how to efficiently use the Flamethrower to it's full potential and you should be fine. If anything I would avoid being in the same line of fire alongside Commando since they still seem to do a pretty good job at CCing until Wave 7+. Then it's up to you on where you should position yourself. Remember to put MAC10 on Semi-Automatic if you want to get those clear headshots and not waste ammo.
Personally, I wouldn't go overboard and try to buy any Pipe Bombs mainly because it may very well be a nuisance to the team and do little damage to when it is needed most. Smokescreens are still double-edged as far as I can tell in Suicidal.
On Normal and Hard I usually go MAC10/Xbow whenever playing with random players and it does not appear people are prepared for Scrakes or FP. Therefore, I end up using the Xbow, albeit the combo is as OP as any other perk going X/Xbow in Normal/Hard difficulty. Here's to hoping they balance the xbow to where you have to go Sharpshooter to make it useful.