1 skiller damage

how much damage % is added per each 1 to all skills grand charm or ring?

There's no simple answer to that. It depends on which skill it is, how high your skill level already is and what other factors affect your damage.

I think for my fire Warlock it was around 4%, when I was comparing various

Opalvein vs. SOJ options.

But it depends on current skill level and other factors. Generally the lower your skill level the higher the pct increase.

a few specifics then

lvl93 cold sorc, blizz damage 15/16k and

Ice Bolt 7k damage
just curious if 2 rings with cold skill damage would deal more damage than 2 rings with 1 to all skills

Well, I just tested it by swapping the actual gear and looking at the character screen. But if you want an answer before swapping the gear:

1) Total your current gear, jewels and sunders that adds % cold damage. Let's say it totals +50% right now. Then a +5% cold damage ring will add (155%/150% - 1) damage, or 3.33% damage

2) +skills doesn't effect synergies, it only impacts the base skill. So use an online table to see what the non-synergized damage % increase is from your current level +1.

For Blizzard, if your current skill level is level 40, then from this table we can see an additional +1 (lvl 41) will take the non synergized damage from: 1770-1839 (1804.5 avg) to 1835-1905 (1870 avg). That represents a 3.6% damage increase.

https://diablo.fandom.com/wiki/Blizzard_(Diablo_II)

Note that for a fire or lightning sorc, you also have to take into account fire and

Lightning Mastery + damage %. But in that case there is no need to calculate - the +1 skill will always be better.