Should I join IIM Shillong? How good is it compared to other older IIMs, especially for finance?

You are asking whether you should take it or not so I will assume that you are planning to take a drop year otherwise. However, preparing and attempting CAT again is not a cakewalk. It requires a lot of determination and motivation and still does not guarantee that you will get a call from an old IIM.

IIM Shillong is rather a good choice for finance. You can contact some of its alumni to get more information on the same. Since your profile is good, Indore is also a good option for you. 

But if you are adamant about joining the old IIMs, then focus on one single goal. Getting maximum percentile in CAT. Start preparing as soon as you decide to drop the year. Anything above 99.7 will guarantee you a call from one of the older IIMs. I have seen many people attempt CAT 3-4 times just to get into their choice of IIMs. 

It all comes down to your determination. Like I said, dropping a year requires a lot of determination, so choose this path wisely. Else consider the various other options you have.