Does 21g seem high for effective tonearm mass?

IMO, the classification of low, medium and high mass arms seems a bit arbitrary. The goal should be to match a given cartridge compliance with the effective arm mass. Plenty of calculators around to help in matching.

"In general higher mass arms control the cartridge instead of the other way around - am I right, Raul ?? "

Interesting way to look at it. While the arm should provide a stable platform for the cart to track, there's a delicate balance between stability and hindrance. These days 21g is considered high mass. Most modern carts are between 15 and 22cu. The Celebration is 12cu and should be a near perfect match, compliance-wise.

Despite the Jelco sales mgr, the Jelco 750d is around 20g, so 21g for the 750e sounds about right. I suggest using as little fluid damping as possible, to preserve transient response. While good results can often be realized with compliance/mass mismatches, you'll experiment at your own risk.
Regards,

Fleib, it's all relative. My highest compliance cartridge is a Denon DL-S1 at 14 cu - the FR-64fx I've got works well with this cartridge mounted in an Orsonic headshell and the small counterweight. Great cartridge for choral and chamber music, etc. When I want more body, I switch out to an Ortofon Synergy SPU and the W250 counterweight, which takes the effective mass up to 30-35 g, (more like an FR-64s). This combination also works well - sounds like a Koetsu with iron cajones !!

For flexibility in compliance matching, it really helps to have a removable headshell arm with light and heavy counterweights - the FR-64fx is still one of the best !

Dcbingaman, the DL-S1, 304, happen to sound best (to me) in arms of greater mass than their compliance/mass match would imply. Such is not the case with most high cu carts, IMO. Many of them will sound sluggish when the excessive eff mass with high moment of inertia, encumbers their performance. Resonant frequencies also go dangerously low.

Yes, it's all relative to the specific cart. Rules of cu/mass matching are a consideration, but not necessarily paramount. Having a removable headshell also adds to the eff mass, adds two more sets of connections, and creates additional arm resonance. Low mass arms tend to be less rigid and resonate with greater amplitude than damped high mass arms like the 750. YMMV. Pick your preference. In this case, OP is using a low cu cart and cu/mass considerations should not be a problem.
Regards,

IMO, you don't want the tonearm to "control" the cartridge. You want the tonearm to allow the cartridge to ride as smoothly as possible through the curvaceous and treacherous grooves of an LP, without bouncing out. At the same time, the tonearm should not impede the cartridge. It's a tricky deal.