Several comments to be made here.
1) Some aquatic vegetation is generally regarded as being good for a pond or lake's fishery. However, having a little hydrilla is akin to having a little cancer, and equally risky if left unattended. Hydrilla is not a plant specie that typically self-regulates its density within the acceptable level of plant coverage. It is an extremely invasive specie in most water bodies, the exception being lakes that experience widely fluctuating seasonal water levels.
2) Herbicides will NOT "eradicate" hydrilla on their own. Due to its various means of propogation, hydrilla is a very tenacious specie. Though it doesn't sexually reproduce, it is quite capable of rapidly expanding its population through fragmentation, and producing turions and tubors - see pics > Hydrilla Pics
. It is the later two structures that are not likely to be impacted by herbicide treatments. Therefore, herbicides are generally considered a temporary control measure (1-5 yrs).
3) In my opinion, LMB populations do not fluctuate as a direct result of hydrilla control. BUT, an indirect result of hydrilla control on public waters may be that catch-n-keep fisherman become more successful due to easier navigation and/or shoreline access. Secondly, the LMB population often becomes scattered - rather than concentrated along the weed lines - making frequent hookups less likely. The net affect of both might make a lake appear to be less populated with fish. In most cases, creel surveys show a dramatic increase immediately after a vegetation reduction. Two popular trains of thought: LMB now have easier access to their forage (hence faster growth-rates), and fishermen have easier access to aggressively feeding LMB (higher catch-rates). Bottom line: LMB populations don't "disappear" in the absence of hydrilla - unless they leave by boat or vehicle.
4) Grass carp on their own, at normal stocking levels, will not bring a severe hydrilla infestion under control. Conversely, stocking too many grass carp may denude the lake of all vegetation for years to come - not a good option. However, using a "hydrilla selective" herbicide treatment followed by a modest stocking of grass carp may be the best plan for controlling and managing the hydrilla population to a more desirable long-term level, or converting the lake's hydrilla mono-culture toward a more diverse and desirable (less invasive) group of plant species.
As for the 22-acre lake in question, the "best solution" requires more details of the lake's unique characteristics. I would be happy to discuss your situation if desired. Otherwise, I would suggest hiring a qualified lake/fisheries biologist in your area to review and advise toward a viable long-term solution to the hydrilla infestation.