Sorry may be a dumb beginners question, but, are single leg guide sizes exactly the same size as traditional snakes?
ie manafacturer recommends #1 snake can I do a straight substitution with a #1 single leg H&H ALS guide?

You can use them if you want the snake dimensions for size 1 are 7.2mm diameter, 7.5mm height, 21mm in length (American pattern)

ALS stand off for same size guide (1) 6.4mm diameter, 7.2mm height & 8.8mm length

So the ALS gives you very slightly less line clearance than snake. The ALS needs only the one thread wrap so minimal weight gain on thread and resin.

The ALS guides are stand off. They are comparatively prone to the fly line tangling around them and very vulnerable to bending from any kind of impact. You'd do better with thinwire snakes as used by Sage (619AP-TW) or fly single-foots (619SL), although the latter are also more tangle prone and bendable than standard snakes. The weight advantages of single foots are overstated, same length of wire, and thicker wire than the TW snakes, longer foot but only one, thus fractionally more thread and finish. In all much of a muchness.