I have heard of this being done, actually... but you do need to discuss it with the Glider DPE ahead of time so they're not surprised. You can hold a Commercial certificate with Private limitations on other categories and classes.
Here's how it works out...
Glider PTS says:
..."hold at least a private pilot certificate issued under 14 CFR 61; "
It does NOT say the Private Certificate must be in Gliders. It says CERTIFICATE. No category or class requirement.
That's based off of 61.123 (h) which says you need only hold a Private Certificate, it doesn't mention category or class.
Your pilot certificate will always be the highest certificate you've earned. If you earn a Commercial certificate in gliders and had a Private certificate in Airplanes, you will then be issued Commercial certificate that will say "Commercial Pilot - Glider" and your limitations section would say "Private Pilot Privileges - Airplane Single Engine Land".
To do the training, you'll need to be current -- INCLUDING a Flight Review whatever your certificate has privileges for today.
Here's why...
Once you're no longer flying on a student certificate, you can't "solo" the glider. You'll (oddly, but correctly) be logging PIC time when doing the required "solo" flights in the glider under the "sole occupant" logging rule. The carve-out for "solo" is so someone without a student pilot certificate can fly by themselves.
You already hold a Private Certificate, but you are not rated in the aircraft to be flown, but you are the sole-occupant. You will log PIC. ( 61.51 (e) (1) (ii) )
And you can't be PIC without a Flight Review.
FAA has been solidly consistent about the Flight Review too, here's a Chief Counsel letter where a Private Pilot asked if he needed a Flight Review when it was expired to solo a glider. They say, yes.
https://www.faa.gov/about/office_org/headquarters_offices/agc/practice_areas/regulations/interpretations/data/interps/2015/newman - (2015) legal interpretation.pdf
And finally...
Aeronautical Knowledge: 61.129 (f) (2)
-----
(f)For a glider rating. A
person who applies for a commercial pilot certificate with a
glider category
rating must log at least -
(2) 200 hours of
flight time as a pilot in heavier-than-air
aircraft and at least 20 flights in a
glider as
pilot in command, including at least -
(i) Three hours of
flight training in a
glider or 10 training flights in a
glider with an
authorized instructor on the areas of operation listed in
§ 61.127(b)(6) of this part including at least 3 training flights in a
glider with an
authorized instructor in preparation for the
practical test within the preceding 2 calendar months from the month of the test; and
(ii) 5 solo flights in a
glider on the areas of operation listed in
§ 61.127(b)(6) of this part.
-----
NOTE: It says "solo flights" here, but it's in context of being the sole-occupant of the aircraft, not logging "solo" time. No one with a Private Pilot Certificate or higher can log "solo" time. Cute, eh?
EDIT: Just remove the above NOTE part. It doesn't apply, see below.