Is it a sin to use Tinkerpanel and putting a fuel tank into another Fuel Tank?

it's not a sin to do ANYTHING in this game. if you have to make it work, use all the tools given in the game

When I first started, I was putting tanks in tanks and rockets in rockets lol. Then I used the tinker panel and it reduced my part count significantly. So if anything, it's good to use it!

You can always reverse the tinker changes you made (if you remember them). Sometimes, I hope they make the tinker panels as powerful as XML editing O.O. But that's another discussion.

PS. Tinker panel is just editing the XML file (e.g. size, weight, etc.).

Welllll........

No

Tinker panel is ok for anything and everything except challenges that say otherwise! It's a sandbox where you can do anything without rules (except when uploading to the website).
 
How detailed and accurate you make the replica is up to you. If you choose to give it additional fuel because it can't do what it did IRL, or what it was proposed to do, I say go ahead! All up to you.