I add fruit to either primary or secondary, or sometimes both; all depending on what I want for the end result. The primary fermentation alters the flavour of most fruits significantly, and you usually lose a lot of flavour & aroma in primary, but I think it's worth it sometimes, just to get that particular flavour difference. For more delicate or expensive fruits I usually put them in secondary. This gives you more flavour & flavour that's closer to the original flavour of the fruit. I made a plum melomel & put fruit in both primary & secondary; this gave me some light, almost "cherry-like/cinnamon-ish" notes in the finished product, right along side the fresh plum flavours from the secondary. If I'd put the fruit just in primary, it would've added that cherry-like/cinnamon-ish note, but it would've had a weaker flavour. If I'd put it just in secondary, it would've been closer to fresh plums, but no depth of flavour from the fermented plums. I can't advise you on the use of tamarind, as I've never used it before, but I'd suggest either drawing a sample & doing some measuring & taste testing with adding small amounts of the concentrate until you find the ratio that best suits you; Or you could try adding it to the mead as you would add it to water (as per directions) or maybe add 1/2 of the concentrate, wait a few months & taste it to see if it needed more or not. Hope you find this info useful, GF.