As a finished volume goal I would target either two five gallon carboys, or a six and a three, or a five and a three. Basically I make the volume fit the carboys I own and I add water at the racking off primary to top off and to create the finished volume.
Montmercy and northStar and Supermont are sour cherries AKA pie cherry. ex. Supermont pH 3.64/ 3.33/ 3.07/ 3.47; SG 1.058/ 1.066/ 1.050/ 1.063; TA 1.63/ 1.76/ 1.97/ 2.06. ex NorthStar pH 3.31/ 3.24/ 3.32/ SG 1.048/ 1.80 TA. Sour cherry has primarily malic acid therefore a sharper taste. (NOTE I have been fighting pH rise with apple which is also malic)
Sweet cherry is primarily citric acid. It is a totally different crop. ex. Ranier pH 4.45 / SG 1.109/ TA 0.16; Bing pH 4.38/ 3.92/ SG 1.086/ 1.094 TA .70/ .97. My feeling is that Bing cherry makes a thin wine. What we normally expect as sharp front notes are missing.
Age? You should have it reasonably clear in four months. I would encourage you to rack to clean carboys which creates some void volume and only bottle that one. At four months I would add KSorbate. I would consider adding Bactiless to both carboys again because it is mainly malic acid.
Tannin? as picked a pie cherry does not have tannin. However as cherry ages some polyphenols polymerize and become tasteable, rough/ low bitter notes. The age developed flavor is reminiscent of choke cherry. At four pounds per gallon this shouldn’t be too bad though.
Did I cover everything so far?