Campaigns and bots can well work together. Campaigns are triggered based on user context and events. When a campaign message is delivered to a user, the user may or may not respond to the campaign message. When the user responds to the campaign, the bot interactions are kicked-off.
Welcome message are meant to help set up context and expectations for a specific messaging channel. They are like a sticky message at the beginning of the conversation and do not interfere with bots or campaigns.