I think that would be a fine idea on both counts. I cannot see a down side to either. In any case I will look at see if I have any resources that might be helpful as I am sure other artists here are approaching new galleries from time to time.
Hi Tony! I think it’s a good idea to mention other galleries you’ve worked with- it shows this is not your first rodeo I like the idea of mentioning other artists the gallery represents- it shows you’re familiar with the gallery and their roster, which is great idea. I’d keep the letter brief. Something I’ve done in the past that has yielded results is to send a small press package via Fed Ex in hard copy- a letter, resume, and 3-4 8x10 images. As more correspondence turns to the digital world, having something to look at that isn’t a screen is nice. And of course they can always visit your website. Best wishes with the gallery hunt!