Should I power up my attacker mons ?

I would for sure power up that Snorlax. Good IVs, a moveset that functions both offensively and defensively (yeah, zenheadbutt is superior to lick but it's a Snorlax with body slam).

The Arcanine also has a great moveset and IVs. Still an excellent attacker even after the nerf. I would power up both.

I bet your Jolteon as is will wreck Gyrados and other water types.

I powered up my Jolteon with same moves a few times weeks ago but left it around 1200-1400. However after the buff, I powered it up to 2000 CP exactly with about 4 or 6 more powerups left for my trainer level. I decided to leave it as is. It wrecks 3000 CP Gyrados and other water types with two Thunders no dodging (lazy).

If you're not hatching eggs constantly to get higher IV mons than yes I would definitely power up all except Jolteon. Eevees are everywhere and you're bound to catch a higher IV one.

Whats you level and whats the CP of the mon's you have listed. These details will determine the answer and without that info its impossible to give you any recommendation. If your Snoralax is under 1000 CP you should transfer it actually not power it up.