However I would like to prepare and try a tail slide to know what it feels like. If my controls are locked Rudder Aileron and Elevators, is it safe to do in the RV's?
I would strongly recommend against playing around with tailslides in an RV. They were not designed for it. Vans would say don't even think about it. Lots of people have damaged their tail surfaces (and flaps) doing either inadvertent, or intentional tailslides...not so much in RVs, but other types. I've never heard of anyone tailsliding an RV, intentionally or otherwise. The risk with the tail surfaces is having them slammed against the stops. But even if you hold the stick and rudder firmly, the tail surfaces weren't designed for reverse airflow. Neither were the flaps. No reason to try it.
But here's what I would recommend to help get a feel for what's happening without tailsliding - tape a piece of yarn about 8" long to the aft end of your wingtip (acro pilots traditionally look at the left wingtip). On the upline into a hammer, watch the yarn. When you get very slow, it'll start to flicker, and if you keep going, it'll eventually go limp and quickly reverse direction. Avoid that. If you get caught with it starting to go limp, the best thing you can do is apply full power (but you should already be at full power) and fully deflect the elevator aft and fully apply left rudder and hold the controls tight. This will cause the airplane to swap ends without too much backward sliding. Pull power as it swaps ends, and then neutralize the controls, or you'll be spinning. Unlikely you'll hurt anything doing this. Have you had advanced spin training? Highly recommended before playing with the edges of the envelope like this.
No need to let it go this far though. Play around with kicking at different stages of the upline, before the yarn really starts flickering hard. You're way late if it actually starts going slack. You'll eventually get a feel for what produces a nice turnaround. Again, one session of knowledgeable ground critiquing will do more for you than weeks/months of playing with this stuff on your own. This yarn thing is just one way to get a feel for things. Once you do, you shouldn't need to keep it on the airplane. The real reason aerobatic pilots have yarn or string (typically tied to their sighting device) is to know the instant the airplane starts backwards so they can apply the elevator one way or the other to cause the airplane to swing a certain way when deliberately doing tailslides. Same thing for reversing aileron on some airplanes when doing torque rolls.
But some people do use the yarn to time the hammer pivot. They kick when the yarn starts to flicker a certain way. There are several ways to gauge the timing of the kick, and some are aircraft-specific. Kicking at 60KT will cause what we call a 'fly-over' rather than a tight pivot. A 180 HP RV at full power has a high enough power-to-weight ratio to make the ASI fairly useless for timing a quality hammer pivot. Some airplanes will buffet at the top due to the tightening slipstream. In the Pitts, I use a certain amount of right aileron deflection (with no roll reaction) to gauge my vertical speed and kick at just the right time (almost dead stopped).
As far as right-rudder hammers go, unless you are flying an engine that turns the opposite direction, they're fairly pointless, and they'll never look as good as a pivot that goes in the direction of slipstream yaw. Going against it is just fighting what the airplane really wants to do. But those who are hellbent on doing hammers in the "opposite direction" will likely need to reduce power slightly. Another thing that helps here is "sliding" your upline a little to provide additional rudder authority at the top. You, for example, would make a very slight bank to the right as you pull vertical. This will require you to make a left rudder yaw correction upon reaching the vertical upline in order to cause the airplane to fly vertically in yaw. This left rudder correction will be held on the way up, and "slide" the airplane to the right. Then when it comes time to kick right, you have additional right rudder authority. I'd recommend forgetting about it, but if you're having fun with those...well that's the whole point.
So, since you've got some negative G capability, try working on doing rolls where you start and end in level flight, with no heading deviation, and your G meter indicates a precise +1/-1G after you're finished and in level flight.