Sounds like a neat lake your family owns, any pictures? How deep is it, bottom substrate? Other than rainbows, any other fish known to be in it already?
Is your family planning on feeding these fish? If so, how many feeders are they planning on running and how much feed do they plan to dispense? I ask this as this can make a substantial difference in how one manages a trout pond or small lake in the case of your family... 7 acres is a good sized body of water and to make sure all the trout in the lake got fed via pellet, you'd probably need at least 3-4 feeders if not more slinging out a substantial amount of feed.
If they don't plan on feeding or only feeding for viewing pleasure say at one dock area, rather than for management purposes management will need to be different. They will be looking at a mostly natural diet for the trout. First this means a much lower stocking rate... Second, many studies have shown that trout, especially rainbows do better in the absence of cyprinids that people often stock as forage. One species the
Utah chub, which is very common in much of the Rocky Mountain area is a major problem to trout populations for this very reason... The cyprinids compete with the trout for invertebrates and zooplankton that make up a large portion of a rainbow trout's diet and even splake and tiger trout, particularly smaller ones. Even larger rainbow trout feed heavily on rather small invertebrates, particularly in a pond or lake. Splakes and tiger trout are more piscavorous, but even they have a large percentage of their diet being invertebrates. I would do everything possible to keep chubs or other cyprinids out of my pond or small lake. In bigger waters, cyprinids may play more of a role but in smaller waters they just are too much competition with the trout. If you absolutely can't stand not stocking some sort of forage fish into the lake, I would recommend the
redside shiner. Unlike the Utah chub which can grow over a foot long, it maxes out at a much smaller size and therefore stays within the range that trout can feed on. It also competes less for food and space with trout... Redside shiners also spawn readily in ponds which is an important attribute for a forage species.
The odds of any trout species naturally reproducing in a 7 acre lake is iffy. With it being spring fed, it does raise the odds but only for some species. Tiger trout are sterile as has been said and splake although not sterile have only been documented to have spawned outside a hatchery environment just a few times with few of the young surviving... If a minor miracle were to happen, for splake to spawn, they'd need clean gravel with upwelling through it. The odds of those conditions is pretty low in a 7 acre lake. For this reason I highly doubt any splake would reproduce in your family's lake. If they did, you'd want to notify a fisheries biologists in Utah as they would love to know the secret!
The only trout I would guess would have a chance are the rainbows if they were able to run up the spring, if it was in the form of a small feeder creek. But with as piscavorous as the splake and tigers are, any baby rainbows would be easy pickings. Throw in the competition those young rainbows would receive from any forage fish stocked and their growth rates would be very slow. Kokanee salmon may be an option and they may be able to pull off a spawn in a 7 acre lake, but again it would depend greatly on several factors.
If it was my pond, I think I would go heavy on the rainbows as has already been done and add a small percentage of other trout as bonus fish. Say 10-20% of the total fish numbers as fish other than rainbow trout. The bulk of your fishery should be rainbows though. If it was my lake, I'd strongly consider some golden color morph rainbows, cutthroats and perhaps a handful of tiger trout and/or splake. Brown trout could be an option but are often very smart and hard to catch. The more piscavorous trout like tigers, browns and splakes should be given more consideration if cyprinids naturally make it into the lake. This would be to help control their numbers which the less piscavorous trout may struggle to do. This is especially important if Utah chubs find a way into your family's lake. Kokanee salmon may also be an interesting option if the lake maintains a cool enough temperature and the lake does not contain any cyprinids. Kokanee are even less adapted to competition with cyprinids as their diet is exclusively made up of zooplankton and small invertebrates. Kokanee are some fine eating fish!
As a side note, I would not mix non trout species in the lake. Trout usually struggle to compete with bass, sunfishes etc. I would keep the lake as an exclusively trout lake.
