The assumptions are reasonable, but I might change a few
- The creatures would all be very complacent and would feed and live
in very vulnerable positions, because they have never learned
They would not feed and live in very vulnerable positions. Rather the definition of vulnerable shifts with the environment. Think about how you left your house this morning. Let's say you left out the front door. Did you open it a crack and take a sniff to make sure there were no predators nearby? Did you listen to the birds chirping to make sure it was safe before turning your back on the world to lock the deadbolt? Were you vulnerable? No. You were balanced within your environment. These animals will be balanced within theirs. It may look to us as though they are vulnerable, but in fact they are plenty safe.
However, complacency will never be seen. There is still going to be competition for resources, and it will be aggressive! Nobody's going to be sleeping on the job on this planet!
- The creatures would mostly be brightly colored, because the pressure to attract a mate would be much stronger than the pressures
to stay hidden.
I think its unclear whether bright colors would be used to attract a mate. Generally speaking, getting a mate to see you is easy. The bright colors are to impress. But why is it impressive? That's harder to say. Its entirely possible the lack of predators may make colors less interesting. With no reason to not be bright and flashy, its not exactly a good tool for judging the fitness of a mate. They may have to find more subtle ways to pick mates.
That's not to say the planet couldn't have bright and flashy animals trying to attract mates, just that it's not a foregone conclusion that they must be that way.
- And finally, the point I am most unsure of. The creatures would quickly acclimate to a newcomer to their area.
What does it mean to acclimate? If I had no predators, and the only thing limiting my ability to live and reproduce is finite resources, I'd acclimate to their presence real quickly: I'd be guarding those resources with all my might. Just because we don't have predators doesn't mean the plants aren't going to develop herbicides to kill off competition, and the herbivores wont aggressively defend their turf. Death is part of the circle of life. Getting rid of predators wont change that.
Could the population be controlled without predators to keep them in
Predators are just one tool evolution has unearthed that limits a population. Fundamentally, its the quest for finite resources which really generates the limits. The competition for resources will be fierce on this planet, and many will die simply because they cannot find the resources they need to continue living.
Two fun case studies. Yeast is a magical compound. Not only does it make beer work, but it has a terribly interesting lifecycle. When yeast enters a fertile area, it multiples like mad in a haploid state which reproduces asexually. This is an extremely fast way to consume the newly found resources. Eventually, the haploids start to starve themselves for resources. With a quorum sensing like behavior, they undergo a transition, combining two haploids into one diploid cell. Once in this form, they undergo sexual reproduction. If the resources continue to dwindle, these diploid cells can form spores, which are hardened against the environment, ready to be transplanted into a more suitable location.
The other case study, Desulforudis audaxviator, is a fascinating bacteria. It is found deep in cracks in goldmines in South Africa. It is known as a "single species ecosystem." Not only does it not have any predators, it indeed is the ecosystem. There is no DNA present in the mines besides that of D. audaxviator. The fact that it can exist in balance tells me that you can find a balance without predators.
Random factoid: D. audaxviator is the only known nuclear powered organism on the planet. One of its several metabolic channels is specialized at using hydrogen peroxide generated by the decay of Uranium atoms nearby.
I think the biggest challenge your ecosystem faces is why there are no predators. With all of the competition for resources, it would be very reasonable for animals to try to take each other's resources. It doesn't take long for that to turn into predatory behavior. You're going to need to come up with an interesting mechanic to ensure predators don't evolve on your planet.