It is great you have pumice! Some ppl grow some succulents even in pure pumice - hopefully one of them will see your question and posts.

I guess mix you suggested could work - but seed starting mix usually contains lots of peat or is 100% peat. Is there ingredients list? Usually, 1st ingredient listed is one there is most of. Sometimes, ingredients are listed by %. I would check that. Problem with peat is that it could get hydrophobic if it dries up. Or, if kept wet, it may stay too wet for too long. But maybe if you use it in smaller amounts it may work???

What do you mean by "new plants"? Are those plants new to you/you just got, or are they newly rooted plants? Which plants do you have?

I use mix of perlite (would definitely use pumice if I could get it easily), and chicken grit in approx. equal amounts. I add between 10-20% of turface, or instead - same amount of soil or cocopeat. You could do something similar. Chicken grit is available in 50lb bags for about CD $12, and usually less in US.

My plants are also outdoors late spring to fall, and indoors, under lights, during cold weather.

Walmart also carries the MG cactus/succulent soil. Although I don't really like Miracle Grow products. Why not try a an independent local garden center rather than the big box stores? Surely they will have some.

@rina, I am getting some new succulents and want to be prepared for them. I have about 40 succulents now and adding few more to my collection. I will post some pics soon.