for modeling worker rabbles, you can always troll through your bitz for those shovels & picks that come on the heavy weapon team sprues and the vehicle up-grade sprue... (hopefully you've got enough to build a few test models?!)
also, as pointed out, dwarves are a good source of 'tool' type weapons. you could also bit order the old orky stick grenades from the stikk bombas set... actually, orky weapons in generall are good because they're crude and rather heavy looking!
for the gas masks, green stuff is your friend... you'll likely want a few catachan models as they have the bare arms which are good for rabble type hoards, so just practic a bit on some spare heads. (you get 7-8 heads on the sprue so trust me, you'll end up with plenty of practice bitz!
)
for the alpha legion squad, most definately! but don't feel they *must* be alpha legion... word bearers and their off-shoot warbands also foster much discord within the imperium's opressed lower class! or else an emissary of tzeentch could be pulling the strings to further some other unfathomable plan?!
and of corse, go for a radically different play style than what you've already got!
cheers!