Your second sentence is not very nice. Many Japanese people who are studying English conversation speak to foreigners like I d like to practice English conversation. But they are not English teachers nor earning any money from it. Instead you should speak to Japanese like にほんご　を　べんきょう　しています。ちょっと　はなしても　いいですか？ which means Can I talk with you for a short time?

I don't know if 私は(better add 今）日本語を勉強しています。一緒に練習してください are non revised sentences. I find these sentences both very natural and polite, nothing wrong with their expression nor grammar. *****We don't say, 小さな質問があります。but （ちょっと）質問があります。お願いがあります。一緒に会話の練習をしてください・してもらえませんか・してもらってもいいですか etc. Well done :D

For me, the best place to start conversations was in onsen. I guess being naked and stuck in a steaming pool together helps. :D I actually didn't have particularly hard time talking to people. I found them extremely friendly and I often got really positive reactions to my Japanese. Just use natural situations, like asking for directions, or if you need to get something read for you, like a bus map or time table. If you're waiting together, or taking the same transportation, you will get a better chance to exchange a few words. Good luck and have a nice trip!