#1
so i've got a small open air gig coming up (a school if you must know)

i have a fender blues junior, and i am very happy with sound, but i think i need more power

would it be better to buy say a deville or attach a cabinet to my blues junior, if the cabinet was attached how loud would this be
#2
how loud is your amp?
Gear

Johnson strat copy
epi les paul jr
epi les paul studio
Ibanez gax 75
crate 10 watt
crate fxt 120
danelectro honeytone
marshall 8200 bichorus
behringer 400watt cab
#4
im not sure if its strong enough to power a 4x12 cab, probably a 2x12, but you should get an awesome sound from it playing full blast, youll really drive the power out of those tubes. if u think its not enough though, get a 1x12 speaker.
Gear

Johnson strat copy
epi les paul jr
epi les paul studio
Ibanez gax 75
crate 10 watt
crate fxt 120
danelectro honeytone
marshall 8200 bichorus
behringer 400watt cab
#5
If you are going to be using cleans at all then go for a Deville or something more powerful. If you want all overdriven then crank it with nother 112 cab under it and it should work. If there is a PA you could always mic it also. If it is outside, then I think you will probably have to mic it.
#6
well maybe it is time to move on up to some higher wattage
Gear

Johnson strat copy
epi les paul jr
epi les paul studio
Ibanez gax 75
crate 10 watt
crate fxt 120
danelectro honeytone
marshall 8200 bichorus
behringer 400watt cab