Hi Cornelius,
I believe you are confusing the numberless multitude that follows the 144,000 (firstfruits) with a second group of 144,000. As far as firstfruits, a person was to search through his field and select the best of the best, that matured first and present them to the high priest at the temple. The 144,000 are called firstfruits because they are the finest, and first to mature of the rest of the crop that follows. If wheat was presented as a firstfruits offering, the harvest that followed would also be wheat. The 144,000 and the numberless multitude that come after them will be the same 'crop'.
Also, Revelation 14 says that they are "not defiled with women" (i.e. do NOT go off track). Not defiled by women is simply an analogy that means they are "faithful" to Jesus Christ, and follow the Lamb whithersoever He goeth. The 144,000 are called firstfruits in Revelation 14:4 because they are the first to be sealed; after which a great harvest, a numberless multitude (the rest of the same crop that matures later) will follow them and will be sealed during the Great Tribulation.
I also believe you are overlooking the big picture regarding the crucial purpose of the 144,000.
Exodus 19:4-6
"You yourselves have seen what I did to Egypt, and how I carried you on eagles' wings and brought you to myself. Now if you obey me fully and keep my covenant, then out of all nations you will be my treasured possession. Although the whole earth is mine, you will be for me a kingdom of priests and a holy nation. . . ."
Priests were those who God called to stand between God and man (Hebrews 5:1). Who was God's chosen kingdom of Priests that were to show (by example) the rest of the world the ways of God? Historically, it was supposed to be the 12 tribes of Israel; but they failed, and Israel was taken into captivity. Later, Paul summarized God's original purpose for the twelve tribes when he said:
Acts 13:47
"For this is what the Lord has commanded us: 'I have made you a light for the Gentiles, that you may bring salvation to the ends of the Earth."
The 144,000 are VERY significant, that is why God chooses 12,000 from each tribe. Originally, God's plan for Israel failed. However, in the end, 12,000 from each of the twelve tribes will be those chosen to accomplish God's purpose and plan....."you will be for me a kingdom of priests and a holy nation. . . ."
The overall plan and purpose outlined in the Scriptures does not support a second group of 144,000, and the fact that one verse uses 'men' rather than 'Israel' is not sufficient evidence to make that assumption when analyzing the whole.