RPE 6 work for someone like me would be quite light. For example my best bench ever is 205 and right now I doubt I could go bench 185. 4 reps in the tank on my bench might be like 120lbs only is that going to be ok?
Yes, absolutely it is ok. RPE is easily scale-able and will be applicable regardless of your strength level. There is no minimum entry level of strength to use the 10/20 methodology. RPE 6 should be super light! Using your numbers, 120 pounds sounds about right.
One piece of advice as you are getting started out: keep it simple, keep the assistance work simple and don’t over think it.
Remember, even if something feels ‘light’ or ‘too easy,’ does NOT mean it’s too light and not working. Most of the gains you will make in your strength training life, will fall within the sweet spot of a 7.5 or so RPE – in the range of 70-85% or 7-8 RPE-ish… Keep this in mind when you feel a 6 is not doing anything.
Same goes for the opposite – even if it doesn’t feel ‘heavy’ it can still be taxing the crap out of your body and eventually hurting you.
Load, strength and corresponding weight with RPE is all relative whether 120lb or 1020lb – to your ability.
For an example, I use 10/20/Life with clients ranging from kids to geriatrics. Since it’s based more on RPE than anything, I can easily assess how fast the speed of the bar (or whatever) is moving and make estimates of where they are. Remember, it’s all relative. Heavy to you is heavy enough, regardless of the weight on the bar. Putting the work in is the important part.