Absolutely nothing wrong with imaging or trying.
First: I am not real familiar with the genetics for long tails.. someone else probably will have more information.. but in case here goes: the long tails are due to at least two genes(multiple sickles, saddles and non-molting).. three ideally but apparently it's very uncommon to find stock with all long tail genes present. Anyways, the long sickles and saddles are more like semi-dominant.. crosses can grow sickles of longer than normal length not as long as on the really good ones. So you are probably most likely to see roosters with fair tail length in the F1.. such as nearly or "just" touching the ground. If that makes you happy then your goal will be relatively easy. If you expect "tail draggers" then that's bit of a challenge but possible.
With projects like this, it's much easier to make progress by using excellent longtail stock. I probably would have done something like silkie x longtail then maybe F1 x F1 and breed the silky feathered F2 back to a pure longtail.. and/or tried my luck at a F1 x pure.. sometimes you can tell which ones are Ff by their looser and softer feathers, not very reliable though.
Color is very much a personal taste. I imagine black silkie & sumatra would be an easy one for a solid colored goal.. however it could be possible even those would be plagued with off color on hackles and saddles. Those would also have the extra feature of having black skin, all-black chickens have been getting a lot of attention lately. A white silkie and recessive white longtail would be another easy one.. probably the easiest one for color in fact.
Other possible challenges are the crest and leg feathering genes.. if your goal is for a crest-less and clean leg, they will be a challenge. Crest is relatively easy to remove from a line(when it's gone, it's gone forever), however leg feathering can be a pain as there are two semi dominant plus a recessive genes for leg feathering. If you like the leg feathering then this will be no challenge.. LOL