in Poland i never met nor heard about religious people who wouldn't be hostile to communism.

Yeah i'm not surprised, most people in France are either hostile or ignorant about communism, because freedom, and since "we live in the least worst system we know".
I suppose that it may be hard two generations later to still find communist people in Poland after the ban of communist symbols and ideas, and having one of the highest growths in Europe.

On coexistence, united in diversity, this a topic that i''ve been very interested in for the last years. First, the middle ages proved that capitalism, or feodalism, or slavery, can subsist with a stagnation, as long as they're powerful enough to maintain their exploitation of the laborers.
Even if that wasn't the case, there're new technologies to report this end of ressources such as thorium-based nuclear reactors, green chemistry, asteroids mining, ...
Your point about ideologies wanting to expand is the most obvious one, i think that a world united in diversity wouldn't stagnate, ideologies should expand slowly and naturally, local laws could enable an initially socialist city/region to become&stay capitalist a very long time, within a socialist country, there shouldn't be a conflict, just personal preferences enriching all of us.
Of course, there's some conditions, i usually talk about a common army, an effective and somehow incorruptible world tribunal to prevent covert actions, very few laws at the world-level but more and more federal->national->regional->communal laws piling upon each other, and world projects, a collaboration between all countries on all levels, shared medias and a celebration of each other diversity, and certainly more, do you see other conditions ?
It's what i wrote earlier about utopias, we need to set the goal in mind and progressively walk towards it, not... simply living, without a clear common goal, so weird not to know the short&long-term objectives of our countries.
What i'm saying is that a set of condition exist so that every country would leave in peace, united yet diverse, without any more reason to worry about their security, and to waste abysmal funds in maintaining a national army, capitalists countries would be protected, socialists as well, islamists, and whatever else survived the colonization.