Technically speaking, they both could be reused.
7 bolt head bolts are torque-to-yield fasteners. General practice is to use them once, and throw them away. Although, according to Mitsubishi, the 7 bolt shank is 97mm originally, and could be reused if they don't exceed 99.4mm.
6 bolt head bolts are not torque-to-yield fasteners, and are completely fine to reuse. Reusing a 6 bolt head bolt isn't stupid unless the person installing it is. Like any piece of metal, it can stretch over time and misuse. And like any bolt that isn't supposed to stretch, if the 6 bolt shank has stretched at all (originally 84mm), then don't use it. Simple. I'd bet maybe 1 out of a set of 10 would measure out of spec after being removed for the very first time. Guys have been reusing these for years without issue. For a stock rebuild, 6 bolt head bolts are more than adequate IMO.
If you want the extra assurance though, ARP studs are a good way to go, especially since the head is already off. The benefit to using studs is they are designed to take more torque than the OEM stuff. If you plan on upping the boost, a little extra clamping force on the head is never a bad thing. But don't be fooled, studs can stretch (even the mighty L19s) just like a stock head bolt, and should be measured before reuse as well.