Ok, so all my numbers before were based on RMU, where it is easy to figure out the sum of the stat modifiers, since the stat bonus is the same as the cost of the stat in point buy. That's not true for RMSS. But let's look at RMSS for a bit.
You are required to have two stats 90+, and no stat can be below 20. You have 660 stat points to distribute. Stats of 90 or less have a cost equal to the stat. Let's assume since you want to have a lower power level that no stat will start above 90. So, two 90's takes 180 of our points. Our minimum of 20 in the other 8 stats takes 160. 180+160 = 340, which leaves 320 to distribute. If we distributed them evenly, that would be 60 in those 8 stats. That's not a very interesting array though.
If we just try to have one of every possible bonus, like this: 90 - 90 - 85 - 80 - 75 - 70 - 50 - 30 - 25 - 20 the total point cost is 615. A bit low.
So maybe 90 - 90 - 85 - 80 - 75 - 70 - 60 - 50 - 35 - 25. That yields stat bonuses of +5 +5 +4 +3 +2 +1 +0 +0 +0 -2.
If we use the fixed potentials, that gives us 96 - 96 - 91 - 91 - 86 - 87 - 82 - 78 - 68 - 64 for stats of +8 +8 +5 +5 +4 +4 +3 +2 +0 +0.
Another alternative would be to have one higher stat: 95 - 90 - 85 - 80 - 75 - 70 - 60 - 40 - 25 - 20 for +7 +5 +4 +3 +2 +1 +0 +0 -2 -3.
Fixed potentials yield 98 - 96 - 91 - 91 - 86 - 87 - 82 - 73 - 64 - 64 for stats of +9 +8 +5 +5 +4 +4 +3 +1 +0 +0.
You can obviously pick lower numbers if you want a low-powered game, but those give stat distributions that meet the expectations of the point-buy system in RMSS. I think the biggest issue is not the starting stats but that RMSS lets you reach your potentials so fast the starting stats are almost irrelevant. RMU only gives you stat gain rolls for two stats each level, so it gets spread out much longer and you have to decide what's important. (I like to have one selected stat and one random stat to keep characters from being too narrow, but that's not the rule.)