Well, a popular mix is 10x caapi 1:1 with spice. Caapi has a range of alkaloid content, but 2% is not uncommon-- and 10x that is 20% by weight. So, if you were to make a mix with 1g Caapi leaves infused with .2g harmala to 1g spice, you should be good to go.

When I make changa, I'll often use a 1:1:2 ratio of calea:blue lotus:spice (electric sheep). It usually results in visuals that are more detailed and refined like intricate interwoven lace than plain spice. It's also triggered Egyptian imagery for me (though I've seen plenty of Egyptian imagery with DMT freebase).

It isn't so much that more will drown it out, but more seems to dull and extend the effects. Harmalas do have a stoning effect on their own, which some people like, me included. So really it is personal preference. My preferred method is to smoke the harmalas sprinkled dry onto herbs and smoked in a regular pipe, then the GVG with only spice when I'm ready.

Herbs will help, but something like the GVG will make it taste nice all the time. It depends on the purity of your extract, but it can actually smell good.