I think it depends on your substrate and the needs of your plants.  i have sand, which has nothing plants needs to grow, so both is not too much.  i don't believe it would harm anything even if it isn't strictly needed.  you can always hold back the liquid fertilizer if you get an algae bloom. that said i also don't think both are necessary to use together unless you have plants that have very high fertilization needs and or poor substrate mixed with RO filtered water like me.

Wait until you add plants

I’m not sure about snails but I have corys in my 75 gallon tank and they are as active or more so after the tabs were introduced about three months ago.

It’s a dark, hard pellet that you place under the sand/gravel near your plants. 

I don't see why it wouldn't. The Flourish tabs are intended to feed root-feeding plants. A smaller plant would only need one every 4-6 weeks I believe, possibly longer depending on the size of the plant(s). I have also read to place the tabs every 6 inches or so for carpeting plants. So depending on the outline of the tank and the number of plants, a 10 pack should last you at least a year.

Well this is a 4 year old question - but for anybody else curious: Eco complete is highly porous and conveys water circulation to the roots just under the surface quite nicely. Tabs could be used in deep beds of eco complete for needy plants OR in deep beds of eco complete that have been exhausted. For a shallow bed of eco complete (2" or less) you will probably find these dissolve into your water column right through the eco complete. Since you can't control how fast they dissolve in a shallow bed, I strongly suggest sticking with the liquid counterpart of this product. If you have a LOT of plants or very NEEDY plants, you would need to supplement even with brand-new eco complete. Note, some plants have the ability most of their nutrients from their leaves, while some plants cannot adapt this way and must be fed directly at the roots - then there's also plants that don't have roots; keep that in mind when considering your tanks nutrient needs.

Yes it will - the undergravel will suck the nutrient through the gravel and pump into the water column. This would be ideal getting pushed down into a very fine gravel. Fine gravel is pretty useless in an undergravel filter. For undergravel filter, use liquid fert so you can control doseage 2-3x a week.

yes you should be fine. the best way to figured out if you will be okay with just roottabs or if you need additional ferts is to look at the roots, lots of thick roots=heavy root feeders.

will be just fine. make sure you insert all they way down though.