I use something similar to the 494RC2 on a monopod (though I've modified it with an Arca-Swiss type quick release platform). It's a little concerning to use it with really big, heavy lenses.... though I've done it and not really had a problem. The 494RC2 appears to be rated to about 8 lbs, which is pretty close to the weight of your lens and camera. For safety, I'd want a head that's rated to at least 2X or 3X the weight of the gear used on it.
Yes, a lot of people like a gimbal mount such as the Nest for use with big lenses like the Tamron 150-600mm. A gimbal works best with lenses that are internal focusing/zooming, don't extend like the Tammy does... but still works pretty well with lenses like the 150-600mm.
However, a full size gimbal like that replaces the existing head entirely and sort of makes the tripod "large lens only". You can no longer use the tripod for "standard" use, with shorter lenses that don't have a tripod mounting ring, unless you swap the tripod head back.
The tripod legset needs to be up to the task, too. I fact, it needs to be adequate to support camera, lens and whatever the tripod head weighs. The Slik U212 appears to be rated to only 5.5 lbs., but that limitation may be the head that it's normally fitted with... I don't see any rating for the legs alone, so it's hard to say what they are capable of supporting. It also relies upon the center column a lot to reach a usable height, which is not ideal for stability. And with a 600mm lens, you need all the stability you can get!
Personally, I use older Gitzo Series-3 Systematic carbon fiber tripods with a heavy duty ballhead, with a gimbal adapter when using longer lenses. The tripods are rated to about 35 lbs. and the heads are rated 40 or 50 lbs, depending upon where you look for info. These tripods also are tall enough that no center column is needed (though I do have a center column on one, but use it reversed for low-angle macro work primarily). A couple of the lenses I use are bigger and heavier than the Tamron 150-600mm, so you might be able to use a little smaller and lighter tripod/head combo... just beware that it's not too small and light.
There are many good, solid tripods out there, besides Gitzo. I'm using these as examples of what you might look for... the models I use are G1325 Mark II and G1348 Mark II (superseded by other models in recent years) and were sort of "gold standard" among wildlife photographers at the time. They still work great.
IMO, it's somewhat false economy buying too light and cheap a tripod and head. You often end up replacing them in fairly short order, looking for more stability and/or because the lighter kit simply wore out quickly with use. To me it makes sense to spend a bit more because a good solid tripod will be more useful, more of a pleasure to use, is likely to hold up better, and may be a once-in-a-lifetime purchase. Another tripod I use is 30+ years old, still does all it's supposed to do... it's just rather heavy, all aluminum, and I ain't getting any younger. So I have pretty much dedicated it to studio use only, now. In general though, it's not like tripods are seeing significant changes and improvements every year or two, forcing frequent upgrades.
The rigs I use would cost upwards of $1500 to buy new, but I've bought two of them used in the last couple years for under $500 apiece, including heads and other accessories.
I prefer a ballhead, which doesn't have protruding levers and arms and can support a lot of weight for it's own size and weight. Pan/tilt heads mostly have protruding control arms. Fluid heads for video can be low profile, but mostly tend to be heavy.
And a heavy duty ballhead can be used in conjunction with a gimbal adapter (such as Wimberley Sidekick, Jobu BWG Micro or Induro GHBA), instead of a full size gimbal head. This allows quickly switching the tripod back and forth for use with long lenses or with more standard-size lenses.
To use a gimbal setup, it's necessary to use the more universal Arca-Swiss style quick releases (rather than Manfrotto) on lens tripod rings and camera bodies. This is because when using the gimbal it's necessary to be able to slide the lens/camera back and forth a little to achieve a good, usable balance that makes the gimbal smooth and effective. The gimbal adapters I mention above also attach to the ballhead using an A-S type fitting. How this works is nicely illustrated at
http://www.tripodhead.com/products/sidekick-main.cfm, even if you don't buy that particular adapter.
An less-well-known additional benefit of these gimbal adapters is that - because they're side-mount - they also can be used to vertically mount the camera itself (with shorter lenses and an Arca-Swiss style plate on the camera body, which is necessary in any case). This eliminates the need for an expensive, bulky "L-plate" on the camera body itself.
Hopefully this gives you some ideas.