In the past five years i found that lemon, lime and etrog seedling are heat tolerant, disease risistant, surivive moderate salty soils and grows fast to become grafting size(etrog is best then lemon then lime). Could i know the pros and cons of such seedlings and if there is better types of seedlings that doesnt have frost threats?

Have you tried any of the better known commonly used rootstocks to compare ? All your selections are in the "Lemon relatives" group of Citrus,
which may drop the fruit quality of sweet scions??? It depends what you want to graft to them, and for what purpose, home or commercial? Are there any others in the "Lemon"group (rough lemon, cuban shaddock etc)which might be more vigorous, or "Lemon"XTrifoliate hybrids such as Citremon more cold hardy ?
Rangpur lime should be highly salt tolerant, and generally hardy. It may be worth trying out.

I havent tried the ones you recommended. I tried valancia, sour orange and mandarin seedling and found that the lemon relatives are more vigourous and heat resistant. Iam home grower which wish to graft juicy oranges to these seedling such as moro blood,valancia, torocco, and navels.
Cold hardiness is not an issue because the temperatures in winter doesn't go below 4c or 39 F.

Lemon rootstocks produce big quantities of fruit, but the fruit quality is always poor. Lemon rootstock is for commercial juice production where they want to emphasize quantity. They then mix with smaller amounts of other juices to get the right taste. An advantage of the lemon stock is rapid growth, it being more vigorous than other stock. However, this also results in poor taste. If there is no danger of freeze or frost, Citrus macrophylla does very well in extremely warm climates, but the fruit it produces is just medium quality. - Millet (708-)

The Meyer lemon is relatively sweet. Can it be used as a suitable rootstock for satsumas/mandarins?

Is there any compatibility issues with using Meyer rootstock?

If you mean Myer Lemon seedling rootstocks, they can be fairly variable from seed, unlike commonly used rootstocks which are highly uniform.
You may find them more interesting to keep, as some variants are highly interesting.

My plan is to topwork a large Meyer lemon into a multi-satsuma/mandarin tree.

When I was at the Citrus Center in Weslaco, I picked up some fallen fruit from what my guide called a "Trifoliate Hybrid". It turns out that it was the 'Bitters Citrange', which according to some, may replace sour orange in the Rio Grande Valley as the rootstock of choice.

Does anyone have any experience with this rootstock? I planted about 20 seeds when I got home.

If it's what I'm thinking it's called c-22.

That's correct. (see previous post subject)